Genetically engineered organisms are useful in a variety of settings. Genetically engineered plants offer more efficient sources of food and fuel. Genetically engineered microorganisms (GEMs) offer unlimited supplies of medically useful proteins and also are of interest in the field of bioremediation.
Bioremediation involves the breakdown of toxic compounds by microorganisms and/or their products. Bioremediation is considerably more attractive than merely transporting wastes, as it offers the possibility of degrading toxic compounds to harmless reaction products.
Bioremediation field trials have involved both in-situ and ex-situ treatment methods. Typically, ex-situ treatment involves the transfer of contaminated waste from the site into a treatment tank designed to support microbial growth, i.e., a "bioreactor". The reactor provides for effective mixing of nutrients and control over temperature, pH and aeration to allow optimum microbial growth.
In-situ treatment involves adding biologicals directly to the waste. This avoids the problems associated with handling (e.g., pumping) toxic compounds. However, in-situ treatment has its own problems. Unlike bioreactors, where microbial growth can be monitored and adjusted, in-situ environmental conditions are difficult to measure and control.
Indeed, it is generally difficult to predict the behavior of genetically engineered organisms in natural ecosystems. There is a concern about the uncontrolled spread of recombinant DNA, including but not limited to the spread of recombinant DNA among indigenous bacterial populations. Potential risk associated with deliberate or unintentional release of GEMs into the open environment can be minimized by the use of debilitated strains. An alternative, and perhaps more appropriate approach is the introduction of conditional or stochastic maintenance functions into GEMs (Molin (1993) Curr. Opin. Biotechnol. 4:299-305; Molin et al. (1993) Annu. Rev. Microbiol. 47:139-166; Ramos et al. (1995) Bio/Technology 13:35-371-3). In such a case, the viability of GEMs depends on the expression of an essential gene or on the repression of a lethal gene controlled by a regulatory promoter responding to changes in the chemical or physical constitution of the environment, or by a promoter undergoing recombinational switches. However, the effectiveness of suicide systems is limited by relatively high frequency of their mutational inactivation, resulting in positive selection of uncontained clones.
Thus, there is a need for better control mechanisms. Such improved approaches should provide better regulation of recombinant gene expression and permit control over the spread of recombinant DNA.